Causal Body

At each death, at the end of each incarnation, this memory-picture has appeared before his soul and left behind what I have called a concentrated essence of forces. So with each life a picture is added. After his first incarnation a man had his first memory-picture when he died; then came the second, richer than the first, and so on. The sum-total of these pictures produces a kind of new element in man. 

Before his first death a man consists of four bodies, but when he dies for the first time he takes the first memory-picture with him. Thus on reincarnating for the first time he has not only his four bodies but also this product of his former life. This is the “causal” body. So now he has five bodies: physical, etheric, astral, ego and causal. Once this causal body has made its appearance, it remains, though it was first constituted from the products of previous lives. 

Now we can understand the difference between individuals. Some of them have lived through many lives and so have added many pages to their Book of Life. They have developed to a high level and possess a rich causal body. Others have been through only a few lives; hence they have gathered fewer fruits and have a less developed causal body.

-Rudolf Steiner 

Kāraṇa-Śarīra:

A Sanskrit term which literally translates as “causal body” and is equivalent to the theosophical use of the term CAUSAL BODY. An alternative term, sometimes used, is kāraṇopādhi (i.e., kāraṇa + upādhi). It is one of three such bodies identified in the Vedānta literature.

It is the reincarnating soul or jīva, retaining within itself the essence of one’s life experiences which cause the conditions of one’s future incarnations.
Since the kāraṇa-śarīra or Causal Body is the receptacle of the essences of a life’s experiences and tendencies, and since in the normal process of REINCARNATION the vehicles which would normally store specific memories and behavioral tendencies are shed, it is only in unusual cases of rapid reincarnation that one would recall a former life. Furthermore, the distilled experiences of a human incarnation could never result in the rebirth in an animal form, so theosophical theory does not admit of the popular Hindu notion of transmigration. Only in cases of the most extreme cruelty or debauchery could future incarnations be considered even a human retrogression. Evolution of consciousness and conscience may be slow for the majority of people, but it is steady.

It is stated in theosophical literature that the kāraṇa-śarīra or Causal Body is formed at the time of Individualization, that is, at the transition from animal to human consciousness.

-Richard W. Brooks 

https://www.theosophy.world/encyclopedia/karana-sarira